Method for continuous measurement of carbon dioxide output during anesthesia. An assessment

Employing the Mapleson D circuit, a modified closed-circuit flow-through technique for the continuous measurement of carbon dioxide production (VCO2) was tested for accuracy and precision in a lung model. The recovery of carbon dioxide production was found to be between 90 and 110%, the maximized errors for a single estimate of carbon dioxide production were between +/- 4 and +/- 28%, with the highest errors at high gas flows and low carbon dioxide inputs. Accepting arbitrarily chosen limits of maximized errors of +/- 10%, it could be shown that the system did not work acceptably when the mean carbon dioxide concentration was below 1.5 vol.% within the fresh gas flow rates (2.2--7.7 1 min-1) and the range of minute ventilation (4--10 1 min-1) employed. The half-life of carbon dioxide washout varied between 41 and 138 s, thus limiting the suitability of the system for detecting changes in carbon dioxide output. The method may be used as an approximate monitor of VCO2 in anaesthetized patients, but cannot be regarded as sufficient for research purposes.